ADDITIVE COCLASTOGENICITY OF SODIUM SELENITE AND CAFFEINE IN CHO CELLS TREATED WITH N-METHYL-N'-NITRO-N-NITROSOGUANIDINE

The clastogenic effect of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in Chinese hamster ovary (CHO) cells and its modulation by Na2SeO3 and caffeine were studied by metaphase analysis of chromosome aberrations (CA) as well as by measuring the formation and repair of single-stran d (ss) DNA breaks employing hydroxylapatite chromatography. Treatment of CHO cells with MNNG (1.25 or 2.5 x 10-5M) for 3 h caused CA in 11 a nd 19% of metaphases scored, respectively. Pretreatment of cells with Na2SeO3 (1-5 mu g/mL) or caffeine (0.2-2.0 mg/mL) for 2 h resulted in a 2-3.5-fold increase of CA frequency. Addition of both modulators dur ing the mutagen exposure tended to cause a slight inhibition of clasto genic activity of MNNG (1.25 x 10(-5)M) or had no effect on CA number when MNNG was used at a concentration of 2.5 x 10-5M. Posttreatment of CHO cells with Na2SeO3 for 20 h after MNNG was ineffective in influen cing the number of metaphases with CA, whereas, at these conditions, c affeine enhanced up to 6-7-fold the clastog,oenic activity of MNNG. Ad dition of both modulators during the whole experiment, 2 h pretreatmen t included, resulted in a further significant increase of CA frequency up to the total pulverization of chromosomes in all metaphases scored . The coclastogenic effect of caffeine was greater in this case. The e nhancement of chromosome-dama,oin,o activity of MNNG by selenite and c affeine was better expressed when this carcinogen was applied at the h igher concentration used. An additive coclastogenic effect was observe d in CHO cells treated simultaneously with Na2SeO3 and caffeine plus M NNG. Ln addition, the treatment of CHO cells with MNNG (5 x 10(-6)M) c aused a rapid increase of ssDNA breaks number reaching maximal values after 30-45 min. However, up to 50-60% of MNNG-induced ssDNA breaks we re repaired during the first 60-150 min after the mutagen exposure. Th e 2 h pretreatment of CHO cells with Na2SeO3 (2 mu g/mL) or the additi on of this trace element after MNNG had no effect on formation and rep air of MNNG-induced ssDNA breaks. The coclastogenic effect of Na2SeO3 in CHO cells treated with MNNG was not directly linked to the inductio n and disappearance of ssDNA breaks measured by hydroxylapatite chroma tography.